In the field of integrated circuit (IC) devices, a speed of being able to turn semiconductor devices and circuits ‘on’ or ‘off’ is known to be an important performance factor. The speed at which a semiconductor switch can be turned ‘on’ and ‘off’ is often related to how fast a gate capacitance of the semiconductor switch can be charged and discharged. FIG. 1 illustrates a voltage switching waveform 100 for a typical semiconductor switch. A driver (not shown) controls a transition time (TRISE and TFALL) 110, 120 of the voltage switching waveform 100 from a low voltage level to a high voltage level. Whilst input/output (IO) speed for modern ICs is increasing, the required load capacitance is barely decreasing. This limits an ability of drivers to decrease this transition time.
A straightforward solution to reducing this transition time is to use termination. FIG. 2 illustrates a simplified termination circuit 200 utilising load termination. Load termination is typically arranged at mid-point of a supply voltage, often achieved by connecting a resistor to a special voltage source, or set using two resistors—one connected to ground and another connected to the supply voltage.
Termination circuit 200 comprises I/O switching drivers 210, 215, operably coupled to respective pull-up/pull-down switches 220, 225. Pull-up/pull-down switches 220, 225 are at least operably coupled to voltage supply 250 and ground 255, respectively. Input/output (I/O) pad 240 is operably coupled between pull-up/pull-down switches 220, 225. A resistive device, in this case resistor 230, is operably coupled between I/O pad 240 and ground 255. I/O switching driver 210 controls, in this example, pull-up switch 220. The resistor 230 causes the voltage at I/O pad 240 to reside at a voltage level between ground and a supply voltage 250, denoted intermediate voltage 260, when I/O switching driver 210 drives pull-up switch 220 ‘high’. This intermediate voltage 260 is dependent upon an impedance ratio between pull-up switch 220 and resistor 230. In order to reverse the voltage transition, pull-up switch 220 is turned ‘off’ by I/O switching driver 210, and, in this example, pull-down switch 225 is turned ‘on’ by I/O switching driver 215, thereby pulling intermediate voltage 260 ‘low’. The switching transition between intermediate voltage 260 and ground is faster than if pull-down switch 225 was switching from supply voltage 250 and ground, as illustrated in voltage waveform 270. Utilising the resistor 230 results in transition times (TRISE and T-FALL) 280, 285 being faster than the transition time 110, 120 from FIG. 1 (for a circuit without resistor 230).
A drawback with the above illustrated example is that resistor 230 consumes current when the voltage at I/O pad 240 is driven ‘high’ and ‘low’. In some cases this drawback may be overcome by using two resistors, one connected to ground and one connected to the supply.
US 2003/0112041 A1, as illustrated in FIG. 3 shows a dual voltage supply circuit capable of switching between two alternatively activatable circuit configurations, supplying ‘low’ and ‘high’ voltages to I/O pad 350. In a first operational mode 300 of FIG. 3, I/O pad 350 is pulled ‘high’ to high voltage supply HVcc by turning ‘on’ both switch 320 connected to high supply HVcc, and switch 310 connected to the ‘low’ supply LVcc. In a second operational mode 305 of FIG. 3, I/O pad 350 is pulled ‘low’, by turning ‘off’ switches 320 and 310, and turning ‘on’ switches 340, connected to HVcc and 330 connected to LVcc. Notably, either high side switches 310, 320 are ‘on’ simultaneously whilst both low side switches are ‘off’ simultaneously, or both high side switches are ‘off’ simultaneously and both low side switches are ‘on’ simultaneously. Hence, a drawback of the circuit of FIG. 3 is that two voltage supplies are required and more power is consumed.